This invention relates generally to electrical musical instruments, and more particularly to a synthesizer for producing tonal effects highly imitative of the speech characteristics of certain reed organ pipes.
One known system for producing tonal effects imitative of the speech characteristics of reed organ pipes is described in Peterson U.S. Pat. No. 4,023,455 and utilized by applicant's assignee in some of its organ models. It produces sound signals simulative of the Waldhorn, Fagot, Trompette, and Post Horn, for example, by applying a train of electrical pulses to a low pass filter having a sharp knee and a very rapid rate of rolloff above the cutoff frequency, the object being that as the frequency of the harmonics of the pulse signal increases, the amplitude of the output signal from the filter remains essentially constant up to the cutoff frequency and thereafter essentially immediately drops for frequencies above the cutoff frequency. The cutoff frequency of the filter is controllable over a range from approximately 2.0 KHz. to 8.0 KHz., the selected cutoff frequency determining the tonal quality of a given organ voice. For example, if a control voltage is set to give the filter a cutoff frequency of approximately 2.0 KHz., the reproduced output signal from the filter sounds very much like an organ Waldhorn, which is a very mellow reed stop similar to the orchestral French Horn. If the control voltage is changed to raise the filter cutoff frequency to approximately 2.8 KHz., more harmonics of the applied signal pass through the filter and the resulting output signal has a tone quality characteristic of the Fagot. As the cutoff frequency is further increased, by even slight amounts, sounds of a strikingly different character are produced as additional high order harmonics are allowed to pass through the filter.
In the Peterson system the cutoff frequency of the filter is varied between predetermined limits by varying the effective resistance of the frequency-determining resistors by connecting light dependent resistors (LDR's) in parallel with the frequency-determing resistors, and varying the intensity of light illuminating the LDR's. The lamp and the LDR's are enclosed in a light-tight container and the intensity of the lamp is varied in accordance with a DC control voltage thereby to adjust and vary the cutoff frequency of the filter. Peterson discloses the use of an incandescent lamp to illuminate the LDR's, and it is also known to use an LED instead of an incandescent lamp, but experience has shown that whichever source of illumination is utilized, this technique for controlling the cutoff characteristics of a filter has shortcomings which make it only marginally effective, in situations such as this, where precise control of cutoff frequency is essential to production of the desired organ voice. For example, because of the practical difficulty of positioning the lamp with respect to the LDR's so as to equally illuminate all of the LDR's (usually four in a package), it is necessary to provide adjustable resistors to balance the resistance values of the LDR's which, in turn, necessitates time-consuming factory adjustments to establish the upper and lower limits of the range of cutoff frequencies, and introduce the risk of inadvertent disturbance of the factory adjustment by a salesman or customer, with the consequence that the resulting organ voice does not correspond to the one selected.
Another disadvantage of the combination of an incandescent lamp and a plurality of LDR's for controlling the characteristics of a filter is that the lamp brightness slightly "lags" both the application of current to the filament and removal of current from the filament. That is, the lamp does not reach full brightness immediately upon application of the control voltage, nor does the illumination abruptly go to zero upon removal of lamp voltage; this leads to a lack of precise timing and control. Moreover, the brightness of the filament of an incandescent lamp decreases with continued use, with the consequence that control of the filter characteristics may deteriorate with time.